The inborn errors of metabolism that are currently under investigation are hyperlysinemia, Lesch-Nyhan disease, maple syrup urine disease, and isovaleric acidemia. In all instances, studies are being conducted on the skin fibroblast growing in tissue culture. Skin biopsies and blood samples (for lymphocyte culture) are obtained from the patients. BIBLIOGRAPHIC REFERENCES: Cox, R. P., Krauss, M. R., Balis, M. E., and Dancis, J.: Mouse fibroblasts A9 are deficient in HPRT and APRT. Am. J. Hum. Genet., 26:272, 1974; Hutzler, J., and Dancis, J.: Lysine-ketoglutarate reductase in human tissues. Biochim. Biophys. Acta 377:42, 1975.